DIFFERENTIAL RESPONSE OF POLYGENIC TRAITS TO MUTAGENIC
TREATMENT IN PEAS

Singh, N.P. and B. SharmaDivision of Genetics

Indian Agric. Res. Institute, New Delhi, India

The five polygenic traits of pea
(days to flowering, pods/plant, seeds/pod, seed size and yield/plant)
studied in the present investigation showed different degrees of response
to treatment with gamma-rays, ethylene imine (EI), and N-nitroso ethyl
urea (NEU) in respect of magnitude of induced variability (Table 1). Seed
yield showed the most induced variation, followed by pods/plant. The
remaining three characters, seeds/pod, seed weight and flowering time,
showed less mutability. Therefore, the two major traits of economic
importance, i.e. pods/plant and yield/plant, are highly amenable to
mutational manipulation. Interestingly, these two traits showed more
induced variability in a positive direction. Therefore, this variability
can be exploited through effective selection.

Despite the similar pattern of
variability in M2 and M3 for all five characters, they differ in the
transmission of variability over generation (Table 1). The increase in
polygenic variability in M3 over M2 often has been referred to as "release
of additional variability". Perhaps, such is the mechanism of induction
and inheritance of micromutations. However, it was higher for pods/plant,
seeds/pod and yield/plant. It seems that such differential behavior of
various characters has not been reported earlier in peas. It is still not
clear whether the character-to-character differences can be attributed to
the intrapopulational structure or previous selection history of different
varieties (1 ). However, such an assumption cannot explain the present
situation, because the untreated material of the same variety did not show
a regular increase in variance with advancing generation.

These results clearly demonstrate
that some characters have a tendency to stabilize sooner than others. This
may be related partly to the
number of polygenes controlling them. From the present discussion, it can
be expected that selection for some characters (e.g. days to flowering and
1OO-seed weight) could be confined to M2 alone, as much advantage is not
expected by advancing the mutagenized population. At least for these and
other such characters both time and labor can be saved, and only M2selections need be advanced to M3 generation for confirmation,
further selection, preliminary testing and multiplication.

The same rule can also be applied to
other characters as well, even though their variance increased appreciably
in M3 over M2. The M2progenies can be classified as promising
on the basis of higher CV and
shift in mean in the desired direction than the highest values of these
parameters in the control, and only these selected progenies may be
advanced for a second cycle of intensive selection. Therefore,
irrespective of whether a character shows increase in variance with
generation advancement or not, preliminary screening in early generations
would be of great help in reducing the volume of work and saving
time.

1. Sharma, B. 1986. Indian J. Genet.
46:88-100.

60

PNL Volume
21 1989 RESEARCH REPORTS

Table 1. Effect of mutagenic
treatment on induced variability (CV%) for different polygenic characters
in M2 and M3 generations.